There is little argument that the chemicals used in fireworks can be hazardous to humans and the environment in large amounts. But is the once-annual dose distributed by Fourth of July fireworks enough to cause serious health problems? And are the ecofriendly fireworks created as a safer alternative really necessary?

The main ingredient in fireworks is fuel (usually made up of metal alloys or organic materials) and an oxidizer, most commonly a perchlorate salt, both of which create the combustion reaction needed for the explosion. When exposed to the high heat created by the combustion, heavy metals in the fireworks produce color. The entire concoction is fired into the sky using a propellant, generally gunpowder.

According to Dr. David E. Chavez, a chemist in the High Explosives Science and Technology division of the Los Alamos National Laboratory, both the perchlorate oxidizer and some of the colorizing heavy metals are toxic compounds. Setting off fireworks (and other pyrotechnic devices, such as military rockets and flares) can release these toxins into the environment.

Although much of the perchlorate present in pyrotechnic devices is transformed into harmless compounds during combustion, any remnants of the chemical that fall back to earth can enter into the soil and water. When ingested, perchlorate is absorbed by the thyroid gland in place of iodine, which can interfere with the production of thyroid hormone, an essential part of metabolism and mental development. As a result, perchlorate exposure may be particularly harmful to fetuses.

A recent study of lake water in Ada, Okla., before and after annual fireworks displays found that the perchlorate concentration in the water increased by up to a factor of 1,000 in the hours after the show, exceeding several states' maximum allowable levels for drinking water. The levels took between 20 and 80 days to return to normal.

The heavy metals that colorize fireworks can also be dangerous, and unlike perchlorate, they're not used up during the combustion reaction. "What you start with is also what you end up with," Chavez explains. "They can get aerosolized and breathed in, or they go into the soil and water." Particularly harmful is barium, used to produce green; studies suggest it may cause respiratory problems, among other maladies. One study found that barium levels in the air increased 1,000 times after a fireworks-heavy Diwali festival in India.

So what does all this mean for the average American picnicgoer? Despite the grim-sounding evidence, experts say that a single Fourth of July event probably does not have a significant polluting effect on individuals. However, the health impact of fireworks becomes problematic when repeated often, or in enclosed spaces. Those most at risk from pyrotechnic-generated pollution are people who are exposed to them regularly, such as theme-park workers.

Based on demand primarily from theme parks and the military--which wants safer flares for its personnel, according to Dr. Thomas M. Klapötke, an inorganic chemist at the University of Munich—scientists have endeavored in recent years to develop more environmentally friendly pyrotechnics.

DMD Systems, a company cofounded by former Los Alamos explosives chemist Mike Hiskey, currently produces a line of fireworks that are perchlorate-free and contain about one-tenth the amount of barium used by traditional fireworks. Hiskey's formulation uses nitrocellulose as fuel and nitrates as oxidizer, thus avoiding perchlorates altogether. This reaction produces only stable gases like carbon dioxide, water, and nitrogen, cutting down considerably on the small particles (mostly metal oxides and potassium chloride) that are released from more traditional fireworks as smoke, he explains.

And just because they're safer, these fireworks aren't necessarily less fun. According to Chavez, ecofriendly fireworks produce less smoke, making colors more visible to the audience. As a result, the amount of potentially toxic heavy metals can be reduced substantially while still producing the same visual effect as more traditional fireworks. Furthermore, these new formulations can create colors, such as deeper blues and reds, that are not possible to generate using old-school pyrotechnics.

While "green" fireworks are cleaner and safer overall, it may be some time before they come to a sky near you, especially while the standards for the chemicals released are still lax and the real health hazards are poorly documented. So far, ecofriendly versions are used in many enclosed venues and in some theme parks that have daily shows, but they're significantly more expensive than their traditional counterparts. And at a time when most fireworks producers are trying to cut costs, Hiskey says cheap and dirty fireworks from China are still typically the pyrotechnic of choice.